Conventionally, as a switching element of a display element, a thin film transistor using a polycrystalline semiconductor film as a semiconductor layer is used.
To manufacture a polycrystalline semiconductor film, a method for polycrystallizing an amorphous semiconductor film by irradiating a laser beam thereon is used. In this method, by forming an oxide film on the surface of the amorphous semiconductor film, it becomes possible to form a polycrystalline semiconductor film by utilizing oxygen contained in the oxide film. On the surface of the amorphous semiconductor film, a natural oxide film is formed through heat treatment and preservation in the atmosphere, etc., of the amorphous semiconductor film, however, this natural oxide film is not stable, so that if the natural oxide film is utilized for polycrystallization of the amorphous semiconductor film, the characteristics of the thin film transistor do not become stable.
Therefore, in order to polycrystallize an amorphous semiconductor film, in the pretreatment process, the natural oxide film on the surface of the amorphous semiconductor film is completely removed with hydrofluoric acid. Thereafter, in the crystallization process, first, the amorphous semiconductor film is irradiated with a laser in the atmosphere of oxygen at a comparatively high concentration, and the surface of the amorphous semiconductor film is oxidized again to form a surface oxide film, and subsequently, the oxygen concentration in the atmosphere is lowered, and by utilizing oxygen introduced into the surface oxide film, the amorphous semiconductor film is polycrystallized.
Thus, for polycrystallization of an amorphous semiconductor film, it is necessary that the surface of the amorphous semiconductor film is oxidized again in the crystallization process to form a surface oxide film after the natural oxide film is completely removed in the pretreatment process, so that there are problems that the number of processes is large and the production efficiency is low.